By Josh Brown, LPKF Laser & Electronics, Tualatin, ORLaser plastic welding, a roughly decade-old assembly technology, includes its own resume of breakthrough applications. From micro-fluidic devices, created with extraordinary accuracy, to cleanroom class V applications, impossible with other welding methods. Laser plastic welding is leaving a noticeable mark in the assembly world. This welding process makes use of a finely-focused laser beam. The part to be welded is comprised of two pieces with dissimilar characteristics: an upper, laser transmissive layer and a lower, laser-absorbing layer. The laser energy projects through the upper part and is absorbed by the lower piece. The absorbed heat melts the plastic at the interface between the two parts and creates a weld.

The benefits of laser welding vs. other plastic joining methods are many: precise heat placement, a clean welding process, unmatched precision and the ability to weld complex and three-dimensional parts.

But, likely, the most important capability of laser plastic welding is quality control from its robust process monitoring systems. The simple fact that laser plastic welding is capable of producing highly-complex parts requires that it also have the ability to monitor the complexities of its own process at a very accurate level.

Process Monitoring MethodsLaser plastic welding often is selected as the joining method of choice for precision and intricacy. Many of the parts in question have high tolerance requirements; therefore, the quality assurance and validation must be equally as precise, able to measure the tiniest deviations from the tolerance abilities boasted by laser welding.

In itself, the process of laser plastic welding is extremely reliable and repeatable. However, the process can be hindered easily by small deviations in the parts to be welded. The two causes for concern are geometric and optical deviations.

There exist five different types of process monitoring techniques for laser plastic welding, ensuring that any part, regardless of its nuances or the variations in the part, are able to be monitored effectively. These five techniques are: Melt-collapse Monitoring, Pyrometer, Reflection Diagnosis, Burn Detection, and Camera-assisted Vision Systems.

Melt-collapse Monitoring. The most robust and often used process monitoring method is collapse monitoring. This technique makes use of the natural convergence of the joining parts as they move together under clamping force.

Typically parts are designed with a collapse rib. This rib once melted and introduced to clamping pressure will collapse. The measurement of this collapse can be used to determine weld quality. The laser welding process itself is highly reliable, but even the smallest deviations in part dimensions can result in poor welds. If the two joining parts are even slightly warped, this can leave gaps. Gaps of more than 0.05mm are known to degrade weld quality significantly. Introducing a collapse rib that is greater in height than the part tolerances can ensure that the distance of collapse will overcome the tolerances.

Once an adequate melt-collapse is determined in testing, parameters are entered into the system. If a part fails to fall within the proper collapse parameters during production, it will be marked for rejection and all data will be stored for later evaluation. The device that measures the collapse is known as a linear voltage distance transducer. It is accurate to less than 0.01mm, which is overkill because even the most precise injection molding processes are incapable of producing dimensional tolerances of less than 0.02 mm.

Pyrometer. Pyrometers measure the temperature within the welding zone. Temperature inconsistencies are directly correlated to inconsistencies in the weld.

Predefined upper and lower temperature limits are developed in testing. If anomalies from burned contaminates or inconsistent part dimensions cause the radiation to fall outside of the defined "temperature envelope," the part will be flagged and the data stored.

Pyrometers typically are used when melt-collapse monitoring is impossible, for example in the case of radial welding of a catheter, where pressure is created via an interference fit and not clamp tooling.

Reflection Diagnosis. This method measures reflected light, but rather than measuring the intensity or temperature, this method measures the divergence of the reflected light from the surface of the part. Parts that are correctly molded fit together perfectly, with no gaps between them; therefore, the only surface of the part as a whole is the upper layer surface. Parts incorrectly developed may have gaps. In such an occurrence, the part would technically have two surfaces: the upper and lower layers. This system is able to compare the divergence of the reflected light. Parts with gapping and, therefore, two surfaces are identified by two peaks of light, whereas properly fitting parts will only reflect a single peak.

Burn Detection. Burn detection is used to recognize surface scorching. Scorch marks typically are caused when the laser strikes a contaminant on the surface of the plastic, resulting in a burn. Such burns emit radiation that is outside of the typical reflected wavelengths and, as a result, can be distinguished. Although scorch marks, usually no more than a few tenths of a millimeter across, rarely have the capability of compromising the weld quality, they often are unacceptable for aesthetic reasons. Camera-Assisted Vision Systems. In some cases, weld quality can be determined by a simple visual inspection. However, being a manual process, this is very impractical and unreliable. Vision systems are capable of monitoring the weld seam automatically and to a much higher accuracy.

Data, Data, DataAll of these methods can be integrated easily into an automation line, where the process monitoring systems communicate with the automation line and failed parts can be discarded immediately. However, inline process monitoring is only half the battle. The data collected from the monitoring of each part can be accumulated and used to even further improve the accuracy of the process and the quality of the resulting parts. A universal and highly-functional software system is capable of integrating data from each process monitoring technique.

The software also is capable of statistical analysis of all data collected. Helping manufacturers determine patterns and the root causes of problems. This data then can be used to adjust the process or make alterations to the actual part. In addition, many automation lines today require part tracking from start to finish. Laser welding system software can be easily integrated into existing tracking systems. When perfect replication from part-to-part is a must, then data is an engineer's most important tool.

Transmission TestingOne of the most important factors for determining viability of laser plastic welding for an application is the transmission rate of the materials in question. Laser welding requires that the upper layer have a minimum level of transmission to allow enough laser energy to the interface for welding to occur.

Laser plastic welding is a very precise process. Small fluctuations in material transmission can affect the consistency of a weld. Variations in transmission rates often are the result of upstream processes like compounding and injection molding. Inline transmission measuring can ensure that each part going into production has the proper transmission rate for a quality weld. Consistency is the key.

List of QualificationsLaser plastic welding is capable of meeting the strictest regulations and engineering practices. The process monitoring techniques provide the foundation for Six Sigma process performance. A short list of laser welding quality certifications includes:

DIN ISO 9001.

ISO/TS 16949 (Quality management systems — special requirements when applying ISO 9001:2008 for volume and spare parts production in the automotive industry).

Good Manufacturing Practices (GMP) — Regulation for quality assurance of production processes and environments for the production of pharmaceuticals, active substances and medical products.

Cleanroom compliance with cleanness class ISO.

Satisfies protection classes IP67 and IP69K.

Laser plastic welding boasts incredible advantages — advantages that are expanding horizons for what is possible in device manufacturing. But laser assembly also is very reliable. No matter how simple or complex an application, laser welding is capable of high levels of quality assurance and repeatability.